In this paper, we describe the immunophenotype and ultrastructure of a new macrophage subpopulation present in inflamed but not in normal human skin. In biopsy material from patients with psoriasis (n = 4), atopic dermatitis (n = 4), and positive patch test reactions (n = 4), spindle-shaped macrophages located along the basement membrane were detected staining positive for CD11c (LeuM5 high, Ki-M1 low), CD68 (Ki-M6 low), and Ki-M8 (low). This subpopulation was most prominent in psoriasis, where a median of 34 macrophages neighbored 100 basal keratinocytes, thus covering about 80% of the dermoepidermal junction. The numbers of atopic and positive patch test reactions were 11 and 13, respectively, accounting for about 20% of the dermoepidermal junction. Transmission electron microscopy revealed an electron-lucent nucleus with dense deposits of chromatin along the nuclear membrane. Interruptions of the integrity of the basement membrane neighboring these cells were also seen. We repeatedly observed parts of basal keratinocytes merging through these openings and macrophages exhibiting cytoplasmic extentions directed toward the porus. Our observations suggest vigorous interactions between macrophages and basal keratinocytes and support the hypothesis that these interactions are an important regulatory event in the pathogenesis of psoriasis.